A large number of light-sensitive materials comprising a support having thereon a light-sensitive layer are known, where the image formation is performed by imagewise exposing the light-sensitive material. Of these, a technique of forming an image by heat development is a system capable of satisfying the issue of environmental conservation or simplifying the image formation means.
In recent years, reduction of the amount of waste processing solutions is keenly demanded in the field of photomechanical process from the standpoint of environmental conservation and space savings. To cope with this, techniques are required to produce light-sensitive heat-developable materials for use in photomechanical process, which can be effectively exposed by a laser scanner or laser image setter and can form a clear black image having high resolution and sharpness. Such light-sensitive heat-developable materials can provide to users a heat development processing system being dispensable with use of solution-type processing chemicals, simple and freed from incurring environmental destruction.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Morgan and B. Shely, Imaging Processes and Materials, "Thermally Processed Silver Systems" A, 8th ed., page 2, compiled by Sturge, V. Walworth and A. Shepp, Neblette (1969). The light-sensitive material used contains a light-insensitive silver source (e.g., organic silver salt) capable of reduction, a photocatalyst (e.g., silver halide) in a catalytic activity amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This light-sensitive material is stable at room temperature, however, when it is heated at a high temperature (e.g., 80.degree. C. or higher) after the exposure, silver is produced through an oxidation-reduction reaction between the silver source (which functions as an oxidizing agent) capable of reduction and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the silver salt capable of reduction in the exposure region provides a black image and this presents a contrast to the non-exposure region. Thus, an image is formed.
This type of heat-developable light-sensitive material has been heretofore known but in many of such light-sensitive materials, the light-sensitive layer is formed by coating a coating solution using an organic solvent such as toluene, methyl ethyl ketone (MEK) or methanol, as a solvent. However, use of an organic solvent as a solvent is not preferred because of its adverse effect on a human body during the production process or in view of the cost for recovery or the like of the solvent.
Accordingly, a method of forming a light-sensitive layer by coating a coating solution using a water solvent free of the above-described problems has been proposed. For example, JP-A-49-52626 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-53-116144 describe the use of gelatin as a binder and JP-A-50-1-51138 describes the use of polyvinyl alcohol as a binder.
Furthermore, JP-A-60-61747 describes the use of gelatin and polyvinyl alcohol in combination. In addition, JP-A-58-28737 describes a light-sensitive layer using a water-soluble polyvinyl alcohol as a binder.
Certainly, when such a binder is used, a light-sensitive layer can be formed using a coating solution comprising a water solvent, and this is advantageous in view of the environmental issue and cost.
However, when a polymer such as gelatin, polyvinyl alcohol or water-soluble polyacetal is used as a binder, the coating obtained has a coated surface of which properties cannot endure the practical use, because the compatibility of the polymer with an organic silver salt is poor. Moreover, the silver tone at the developed area becomes brown or yellow and quite differs from black which is regarded as a proper color and preferred, or the blacking density in the exposed area is low and the density in the unexposed area is high, thus, the commercial value is seriously impaired.
European Patent 762,196 and JP-A-9-90550 disclose a technique of incorporating Group VII or VIII metal ion or metal complex ion into a light-sensitive silver halide grain for use in a heat-developable light-sensitive material and incorporating a hydrazine derivative into the light-sensitive material, whereby a high-contrast photographic property can be obtained. If a binder for use in the above-described water solvent-type coating solution and a nucleating agent such as hydrazine are used in combination, a high-contrast image may be obtained but a problem arises at the same time such that fog is readily generated. The fog disadvantageously increases with the passing of the time.
On the other hand, the semiconductor laser technique abruptly growing in recent years has made it feasible to miniaturize an image output device for use in the medical art. Naturally, techniques have been developed for an infrared ray-sensitive photothermic silver halide photographic material which can use a semiconductor laser as a light source. In this respect, spectral sensitization techniques are disclosed in JP-A-3-10391, JP-B-6-52387, JP-A-5-341432, JP-A-6-194781 and JP-A-6-301141 and antihalation techniques are disclosed in JP-A-7-13295 and U.S. Pat. No. 5,380,635. The light-sensitive material contingent on infrared exposure can be greatly reduced in the visible ray absorption of the sensitizing dye or antihalation dye and a substantially colorless light-sensitive material can be easily produced.
However, the dye which absorbs an infrared ray and undergoes spectral sensitization generally has a high HOMO and a strong reducing ability and reduces silver ion in the light-sensitive material, as a result, the fog of the light-sensitive material is liable to grow worse. In particular, storage under high temperature and high humidity conditions or storage for a long period of time may cause conspicuous changes in the performance. If a dye having a low HOMO is used so as to prevent deterioration in the storage stability, the LUMO becomes low in turn, as a result, the spectral sensitization efficiency is reduced to give low sensitivity. Such problems with regard to the sensitivity, storage stability or change in the performance not only arise in a wet photographic light-sensitive material but also become more serious in a heat-developable light-sensitive material.
Under these circumstances, a technique of providing a heat-developable light-sensitive material having photographic capabilities such as good coated surface property, good silver tone at the development, high contrast, low fog and good storage stability, as an aqueous light-sensitive material advantageous from the aspect of environmental issue and cost, has been demanded.